1. Field of the Invention
The invention relates generally to the semiconductor power devices. More particularly, this invention relates to an improved and novel manufacturing process and device configuration for providing the MOSFET device with shielded trench gates with cells provided with Schottky source contact for improving performance of high frequency power switching, H-bridge and synchronization rectification applications.
2. Description of the Related Art
In order to reduce the power consumptions and to increase the switching speed of a power semiconductor power device, it is desirable to further reduce the on-resistance and the gate capacitance. Integration of an Schottky diode in a semiconductor power device such as a metal oxide silicon field effect transistor (MOSFET) has been implemented. FIGS. 1A and 1B show the standard MOSFET devices that integrate the Schottky diodes to bypass the body diode thus improving the behavior of a MOSFET device. Improvements in the MOSFET device performances enhance the H-bridge and synchronization rectification applications. Specifically, FIG. 1A shows FIG. 1A shows a MOSFET with an integrated Junction Barrier controlled Schottky (JBS) area. The integrated JBS may be a Schottky diode array with a P-N junction grid interspersed between the Schottky contacts. The P-N junction will pinch-off the channel regions under the Schottky contacts to inhibit the formation of large reverse leakage current once a threshold reverse-biased voltage is applied. The shielding effect caused by the depletion layer also improves the breakdown voltage. However, there is a tradeoff that there is an increase of the series resistance. Also, since the presence of P-N junction in the integrated JBS regions takes up a large portion of surface area, for practical considerations, it may be required to reduce the overall Schottky contact areas dedicated to the forward conduction. Under the circumstance, there is an increase of the on-state forward voltage cause by this reduction of the overall Schottky contact area. In FIG. 1B, the integrated trench MOS barrier Schottky (TMBS) is implemented. The integrated TMBS includes Schottky diode array interspersed with MOS trench. The charge coupling between the majority charge carrier in the mesa-shaped portion of the epitaxial/drift region and the metal on the insulated sidewalls of the trenches causes a redistribution of the electric filed profile under the Schottky contact which improves the breakdown and reduces reverse leakage current.
U.S. Pat. No. 4,675,713 discloses a method of using the source Schottky junction as the body contact for a semiconductor power device. U.S. Pat. No. 4,983,535 discloses a fabrication method to manufacture a DMOS device with a source implemented with a refractory metal Schottky barrier located on top of the body region. However, these devices still have the limitations of using metals of relatively high barrier height. The device performance cannot satisfy the modern applications that require further reduction of the on resistance and higher drive currents.
FIG. 2 shows an improved DMOS submitted as a co-pending application by co-inventors of this patent application. The DMOS has an improved configuration. Specifically, in proximity of the gate trench and adjacent to the source, there is a source-body contact trench with an anti-punch through implant disposed along the trench wall. An integrated Schottky diode is formed by depositing a high barrier height metal at the bottom of the source-body contact trench to function as the integrated Schottky contacts. A low barrier height metal is further deposited overlaying the high barrier height metal to provide ohmic contact for source and body. The DMOS device as shown in FIG. 2 provides the advantage of integrating a Schottky in every cell with no loss of die active area to form such a Schottky as in older approach. However, the high barrier height metal as required to achieve acceptable low leakage current in the off state presents a disadvantage due to the higher costs of depositing both the high barrier height metal and the low barrier height metal to meet the requirements of Schottky and source-body ohmic contact.
Additionally, the above device configurations as shown in FIGS. 1A, 1B and 2 are still limited by a relatively high gate-to-drain capacitance (Cgd) caused by the coupling between the trenched gate and the drain disposed on the bottom portion of the substrate. For power transistors including MOSFET and IGBT, new device configurations and manufacturing processes have been disclosed implemented with different gate structures including a shielded gate trench (SGT) structure to reduce the speed-limiting capacitance between the gate and the drain of these switching power devices. The technologies as disclosed are still confronted with technical limitations and difficulties. As there are growing demands for high frequency switch power devices, an urgent need exists to provide effective solutions for resolving these technical difficulties and limitations.
Baliga discloses in U.S. Pat. No. 5,998,833 a DMOS cell as shown in FIG. 1C. A source electrode is placed underneath the trenched gate to reduce the gate-to-drain capacitance. The gate of the DMOS cell is divided into two segments. The gate-to-drain capacitance is reduced because the contributions to capacitance from the gate-drain overlapping areas are eliminated.
In U.S. Pat. No. 6,690,062, a MOSFET device as shown in FIG. 1D is disclosed where the switching behavior of a transistor configuration is improved by providing a shielding electrode in an edge region. The shielding electrode surrounds at least sections of an active cell array. There is a capacitance between an edge gate structure and a drain zone. The shielding electrode located in the edge region reduces the gate-drain capacitance CGD of the transistor.
In U.S. Pat. No. 6,891,223, Krumrey et al. disclose a transistor that includes transistor cells disposed along trenches in a semiconductor substrate with two or more electrode structures disposed in the trenches. Furthermore, metallization structures are disposed above the substrate surface as shown in FIG. 1E. The trenches extend into an inactive edge region of the transistor. An electrical connection between the electrode structures and corresponding metallization are established in the edge regions.
The above patented-disclosures including transistor configurations still have a common difficulty. The source electrode disposed on the trench bottom is connected to the source voltage through an edge area of the semiconductor power device. This inevitably increases the source electrode resistance. Furthermore, the extra masks needed to create such connection also increase the cost of manufacturing.
Therefore, a need still exists in the art of power semiconductor device design and manufacture to provide new manufacturing method and device configuration in forming the power devices such that the above discussed problems and limitations can be resolved.